1. Field of the Invention
The present invention relates to a motorboat propeller, and more specifically to a high-speed motorboat propeller formed with a cylindrical boss through which engine exhaust gas is educed directly into water.
2. Description of the Prior Art
Conventionally, high-speed motorboat propellers formed with a cylindrical boss through the inner hollow space of which engine exhaust gas is educed directly into water have been well known, because these propellers have such advantages that exhaust sound can be reduced; exhaust smoke will not be seen; and exhaust pipe end will not be contaminated by soot.
FIG. 1(A) and (B) show a first example of prior-art motorboat propellers formed with the exhaust boss, in which the propeller comprises a boss 2 and three blades 3 fixed to the outer surface of the boss 2 at regular angular intervals. Further, an exhaust passage 4 is formed within the boss 2 to educe engine exhaust gas introduced from an engine into water from a rearmost end of the boss 2.
FIG. 1(C) is a development view taken along the outer circumferential surface of the boss 2, in which water 12 flows against the blade 3 at an elevation angle .alpha. with respect to the blade 3 when the blade 3 is assumed to be fixed. In FIG. 1(C), Z.sub.p1 denotes a deviation pressure distribution domain in which negative (-) and positive (+) pressure (deviated from the roughly atmospheric pressure) is produced by water 12 flowing against the blade 3. In FIG. 1(C), when taking into account exhaust gas flow 14 along a generatrix G1 on the outer surface of the boss 2, there exists a problem in that the propulsion efficiency is reduced, because exhaust gas 14 educed from the rearmost end of the boss 2 through a point 13 flows upstream into the negative pressure (vacuum) domain near the outer surface of the boss 2 and further is sucked into a back surface 3a of the blade 3. In this connection, exhaust gas flow 14 along another generatrix G2 on the outer surface of the boss 2 through a point 15 can smoothly flow downstream. In summary, there exists a problem in that exhaust gas flow 14 passing through the negative pressure domain produced by the blade 3 flows upstream along the outer surface of the boss 2.
FIGS. 2(A) and (B) show a second example of prior-art motorboat propellers formed with an exhaust boss, in which a trumpet-shaped portion 5 is formed at the rearmost end of the boss 2 in order to reduce exhaust gas pressure at the boss rear end. In this example, since water flows against the trumpet-shaped portion 5, an additional annular positive pressure domain Z.sub.t is produced as shown in FIG. 2(C), so that it is possible to reduce the area of the negative and positive pressure domain as designated b Z.sub.p2, as compared with that Z.sub.p1 shown in FIG. 1(C). In other words, it is possible to prevent the exhaust gas flow 14 along the generatrix G1 from flowing upstream, and additionally to reduce the exhaust gas pressure at the rear end of the boss 2, as compared with that of the first example shown in FIGS. 1(A) and (B).
In this second example, however, there exists another problem in that the trumpet-shaped boss end 5 increases fluid resistance or propulsion resistance and therefore the propulsion efficiency is deteriorated.
FIGS. 3(A) and 3(B) shows an example of prior-art motorboat propeller provided with a boss cap 6 instead of the hollow boss. In the propeller provided with the boss cap 6, since there exists a problem in that a vortex 8 is readily produced behind the boss 2, the boss cap 6 is formed with a plurality (four in FIG. 3(B)) of (fluid flow) straightening fins 7 which guide fluid flow in such a direction that the vortex 8 produced behind the boss cap 6 can be reduced in order to increase the propulsion efficiency. However, the prior-art propeller formed with the fins or subblades 7 as shown in FIGS. 3(A) and (B) is different from the prior-art propellers formed with a cylindrical hollow boss which produces less vortex in general.